1. Field of the Invention
The present invention relates generally to water heaters and boilers, and more particularly, but not by way of limitation, to a control system for a modulating water heater or boiler which is particularly constructed for use as one of a plurality of water heaters or boilers controlled in a cascading sequence.
2. Description of the Prior Art
Conventional water heater technology utilizes a burner designed to operate at a fixed flow rate of combustion air and fuel gas to the burner. Such a water heater cycles on and off in response to a control system which monitors the temperature of water in a storage tank or elsewhere in various conduits of the water supply system. One example of such a typical prior art system which is presently being marketed by the assignee of the present invention is that shown in U.S. Pat. Nos. 4,723,513 and 4,793,800 to Vallett et al., the details of which are incorporated herein by reference.
It has been recognized that, in circumstances where there is a substantially varying demand for heat input to the water supply system, greater energy efficiencies can be achieved through the use of a water heater which is capable of operating at different energy inputs. One example of such a system is that sold by Lochinvar Corporation, the assignee of the present invention, under the trademark COPPER-FIN II®. The Lochinvar COPPER-FIN II® system utilizes a plurality of staged burners which can be brought on-line or taken off-line as the demand for heat energy changes. The COPPER-FIN II® unit includes multiple banks, for example, first, second, third and fourth stages. It initially turns on all four stages of burners, and as it approaches the desired temperature, it sequentially shuts off units to decrease the input energy. This type of system provides variable input, but it is not continuously variable. Instead the input can be changed only in substantial increments corresponding to the heat output of one burner stage.
The prior art has also included proposals for water heaters having continuously variable output over a range of outputs. Two such systems are shown in U.S. Pat. No. 4,852,524 to Cohen and U.S. Pat. No. 5,881,681 to Stuart. These systems, which have been marketed by Aerco International, Inc. under the Benchmark name, utilize a nozzle mix burner which receives independent streams of combustion air and fuel gas. A fuel/air valve is utilized to simultaneously control the flow of air through the air line and fuel through the fuel line so as to provide a varying input of fuel and air while maintaining a constant fuel to air ratio. The blower speed remains constant on these systems.
More recently the assignee of the present invention has developed a continuously variable water heater with variable air and fuel input, as shown in U.S. Pat. No. 6,694,926 to Baese et al. In the Baese apparatus a variable flow blower provides premixed combustion air and fuel to the burner at a controlled blower flow rate within a blower flow rate range. This allows the heat output of the water heater to be continuously varied within a substantial flow range having a turndown ratio of as much as 4:1.
Various systems for controlling a plurality of modulating boilers are described in U.S. Pat. No. 5,042,431 to Shprecher et al., including systems in which the last boiler turned on is the modulating boiler.
In large commercial operations it is common to utilize a plurality of commonly controlled heat exchangers such as those of the Vallett et al. patents or the Baese et al. patent described above. A number of unique problems are encountered when using such heaters in groups, and the present invention is directed to improved control systems for such boilers.